In an emergency, a person's mobile device (i.e., user equipment (UE)) is often his or her lifeline for help. This dependence makes it crucial that the mobile device be able to establish emergency calls (e.g., to 9-1-1) whenever necessary. To this end, mobile devices are generally allowed to establish emergency calls through cells of wireless networks that they are not authorized to use for normal service.
In a Long Term Evolution (LTE) or Universal Mobile Telecommunications System (UMTS) radio network, a mobile device establishes a radio resource control (RRC) connection through a cell to use network services. The mobile device can camp on a suitable cell to obtain normal service. The mobile device can camp on an acceptable cell only to originate emergency calls (e.g., a circuit switched (CS) or an Internet Protocol Multimedia Subsystem (IMS) emergency call) and receive Earthquake and Tsunami Warning System (ETWS) and Commercial Mobile Alert System (CMAS) notifications. As a result, a mobile device establishes an emergency call through either a suitable cell or an acceptable cell.
Like any other call, emergency calls can be disrupted by failure conditions that interrupt the RRC connection. Failure conditions may include radio link failures, handover failures, mobility from evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access (e-UTRA) failures, integrity check failures, RRC connection reconfiguration failures, or other failures. To continue the emergency call after a failure condition, the mobile device can perform an RRC connection reestablishment procedure in an LTE network or a cell update procedure in a UMTS network. Existing mobile devices operating in an LTE or UMTS network with an established emergency call via an RRC connection often struggle to reestablish the RRC connection once it experiences a failure condition.